The present invention relates to lasers and optical pumping of lasers and, particularly, to a technique for diode pumping a dye laser.
Dye lasers, and particularly organic dye lasers, have certain unique features. Depending on the specific dye in the laser, the output wavelength is tunable over a bandwidth of approximately 100 nanometers. Dye lasers can operate from the ultraviolet to the infrared, and a single laser resonator cavity can be used to cover this entire wavelength range simply by changing dyes and coatings on the intracavity optical components as appropriate.
The first dye laser was invented in 1966. See, for example, P. P. Sorokin and J. R. Lankard, IBM Journal, Vol. 10, p. 162, 1966. This dye laser, as all lasers, consists of three main parts: the pump source (generally an optical source such as a laser or flashlamp); the gain element, also called the laser medium, the dye-containing gain element or the laser gain element; and an optical resonator, composed of end reflective elements such as mirrors that are optically aligned to resonate the intracavity optical flux. Other components can be introduced in the resonator, such as wavelength tuning components or lenses.
In a dye laser the pump energy originates with an optical source, either from flashlamps or another laser. The laser medium or dye-containing gain element may be an organic dye dissolved in a liquid solvent such as ethylene glycol, although the "solvent" can be a solid state host for the dye such as a plastic, glass, thin film or gel. The dye concentration is relatively low, ranging from approximately 10.sup.-3 to 10.sup.-5 molar. The optimum concentration depends on many factors, including the pump wavelength, absorption length of the gain medium for the pump flux, the desired output wavelength, the gain and loss of the dye at the laser wavelength, the dye flow rate and the resonator configuration. Interactions between the dye and solvent can affect the efficiency and gain at a specific laser wavelength.
The dye molecules are excited by absorbing pump light. Part of this energy is released in the form of radiation. Spontaneous emission, or fluorescence, is one form in which radiation may be released. However, under appropriate conditions, the radiation loss occurs through stimulated emission. In this case, the emitted light is coherent. Stimulated emission occurs when the laser medium is contained in a suitably designed resonator. The feed-back of the emitted radiation stimulates even more radiation, and with each pass optical amplification occurs. In such a case the resonator is said to produce laser emission.
A suitable pump source for exciting a given dye laser must be one that emits radiation that can be absorbed by the dye in the gain element. The absorption wavelengths are determined by the specific nature of the dye. Most laser dyes absorb at wavelengths slightly shorter than the emission wavelengths. The shift between the wavelength corresponding to the absorption maximum and that corresponding to the emission maximum is sometimes referred to as the "Stokes shift". For dyes that emit between 700 nm to 800 nm, the absorption bands are typically between 600 nm and 700 nm. These dyes are generally pumped at 647 nm by a krypton ion laser. Dye absorption bands are structure-less and broad, spanning a region of approximately 100 nm. In addition to the Stokes shifted absorption band, many dyes also absorb in the UV. UV pump lasers include nitrogen and excimer lasers such as KrF or XeCl lasers. Excimer pumped dye lasers are capable of high operating efficiency and high repetition rate.
Several years after the first demonstration of a pulsed dye laser, the first continuous wave (cw) dye laser was demonstrated. See, for example, O. G. Peterson, S. A. Tuccio and B. B. Snavely, Applied Physics Lett., vol. 17, p. 245, 1970. This laser was pumped by an argon ion laser.
There is an enormous versatility to the operating modes that a dye laser is capable of. This versatility has allowed the dye laser to continue to be a vital optical tool for research, medical, commercial and military applications over the decades since it was first developed. These modes of operation are too numerous to list here completely, but a concise listing are: tunable cw output from 400 nm to 1 .mu.m, stable single-mode operation with linewidths less than 1 kHz; tunable pulsed operation from 320 nm to 1.2 .mu.m, conversion efficiencies (pump power in to tunable dye laser power out) exceeding 50%, extremely high pulse energies including an output of over 100 Joules in a single pulse; ultra-short pulses as short as 27 femtoseconds (fs); high average power exceeding 1 kW and broad bandwidth.
When first introduced, dye laser operation could not be duplicated by other sources. No other class of lasers could produce tunable cw or pulsed radiation at comparable power levels. However, in recent years developments in the generation of tunable solid state lasers and nonlinear optical conversion techniques have greatly reduced the operating regime for which dye lasers are the most appropriate source. In particular, the introduction of the Ti:sapphire laser in 1985 has virtually eliminated the use of dye lasers at wavelengths greater than approximately 725 nm. The Ti:sapphire laser operates both cw and pulsed. As a cw laser, Ti:sapphire is often pumped with an argon ion laser while as a pulsed laser it is typically pumped by the second harmonic of a Nd:YAG laser. Organic dyes do not operate efficiently above 800 nm, and for this wavelength region Ti:sapphire is an ideal replacement for the dye laser. In addition, the convenience of a solid state laser is a compelling advantage compared to a dye laser. Dye lasers typically contain dyes dissolved in an organic solvent. Many solvents are typically used and include ethylene glycol, propylene carbonate, dimethly sulfoxide (DMSO), methanol, ethanol, water, and numerous others. Dye solutions must be mixed and the dye solution must flow through the excitation beam to prevent heating and mitigate losses induced by photochemically degradation of the dye.
Another area where dye lasers have been challenged is in the production of pulsed radiation throughout the visible spectrum. The recent introduction of optical parametric oscillators (OPO) pumped by the third harmonic of a Nd:YAG laser at 355 nm produces tunable, efficient operation from approximately 450 nm out to 4 .mu.m. The OPO material is solid state, providing the user with the convenience of a solid state material. This is in contrast to the required dye flow plumbing and dye mixing inherent in most dye lasers. However, the OPO is limited to several hundred mJ per pulse and average powers of only several Watts. For higher average power or for higher energy per pulse the dye laser is a more suitable approach. Also, OPOs produce short (approximately 10 ns) pulses, which may be unsuitable for certain applications. For example, transmission of the pulse through an optical fiber might produce optical damage to the fiber due to the high peak power. In this case a longer pulse is desirable.
For tunable long pulse or cw emission from the near UV up to approximately 750 nm the dye laser is still the optimum choice. Dyes are inexpensive, especially compared to the cost of manufacturing a solid state laser gain element. For high repetition rate, tunable dye lasers demonstrate high efficiency and are among the most convenient to use.
The recent trend in dye lasers in the commercial environment is one that emphasizes dye laser convenience while complying with governmental regulations controlling the use, exposure and disposal of dyes and their organic solvents. To this end, two notable developments have recently been introduced. The first, used in several medical laser applications, is the dye cartridge. The cartridge contains a highly concentrated premixed dye solution. The dye flow system typically contains filtration units that remove depleted dye from the solution. The operation of this type of laser allows much greater operating lifetime. Part of the dye solution (about 10%) is continuously circulated through the filtration unit to remove depleted, or in some cases all, of the dye. Simultaneously, fresh, highly concentrated dye solution is injected back into the flow system. In this manner a small volume of dye solution can last for many hours of uninterrupted use. Once the dye cartridge is depleted, it is replaced with a new one, and the filter is replaced as well. The operator does not have to touch or mix the dye.
A more interesting development has been the introduction of dye-impregnated plastics. The plastic host, typically polymethyl methacrylate (PMMA), replaces the solution in a dye laser and allows the convenience of a solid state gain element at only a fraction of the cost. In addition, the advantages of the dye laser, including the high tunability, the wide wavelength range, the ability to operate in a number of different modes (cw, high repetition rate, long pulse, etc.) and the low cost of the gain element, are maintained when using a plastic impregnated with a dye as the laser gain element. Other types of solid state dye laser gain elements have been developed including dyes in glasses, dyes in gels and thin films of dyes.
Overall electrical efficiency is an important consideration for lasers operating in an environment where primary power is limited. For laser pumped lasers, the overall electrical efficiency (.eta..sub.e) is determined by the product of the electrical-to-optical conversion efficiency of the pump laser (.eta..sub.p), the efficiency of the relay optics used to transport the optical pump flux to the laser gain element (.eta..sub.t) and the optical conversion efficiency (.eta..sub.o). The optical conversion efficiency is the ratio of laser output power to pump power incident on the gain element. For cw dye lasers, .eta..sub.o can exceed 50%, but the electrical efficiency of the pump laser is generally quite low. For ion lasers, .eta..sub.p is typically 0.05%, exclusive of cooling. Cooling is required to remove heat from the ion laser and generally consumes a great deal of power and/or tap water. One method of producing higher .eta..sub.p is to pump with a cw, laser diode-pumped, doubled Nd:YAG laser. However, the cost per Watt of a diode-pumped, doubled Nd:YAG laser is substantially higher than that of an ion laser. In addition, 532 nm is an ineffective wavelength for pumping near-IR emitting dyes.
Another method of increasing .eta..sub.p is based upon using laser diodes directly to pump the dye. A patent by G. Wang, U.S. Pat. No. 3,890,578, describes a waveguide laser in a cell pumped by pulsed AlGaAs laser diodes emitting at 820 nm. The dye in the waveguide laser cell is IR 140. The diodes were pulsed at 50 .mu.s. However, the concept demonstrated by Wang cannot easily be scaled to higher pump power or to other types of dye cells. In addition, it is not clear that this type of configuration would work for a cw laser. A report of direct diode pumped dye lasers appeared in 1987; see, for example, O. V. Bogdankevich, M. M. Zverev, E. M. Krasavina, I. V. Kryukova and V. F. Pevtsov, Soviet Journal of Quantum Electronics, vol. 17, p. 133, 1987. In this work, an e-beam excited visible semiconductor laser was used to excite rhodamine 6G and coumarin 47 dyes. While this work is of interest scientifically, there is little possibility that these blue-green semiconductor lasers will be available at practical power levels when pumped by more conventional sources. E-beams are generally large, high-voltage sources and are inconvenient for most purposes. In addition, these semiconductor lasers are also pulsed, and it is not clear that this technique would work cw.
A cw diode-pumped dye laser was described that is based on a commercial ion laser-pumped dye laser; see D. P. Benfey, R. E. Boyd, D.C. Brown, J. C. Watkins, W. J. Kessler, S. J. Davis, C. Otis and L. Pedulla, Proceedings of the SPIE, vol. 2115, p. 204, 1994. Because these authors did not design the laser resonator, but instead used a commercial laser that was designed to be pumped with an ion laser, they were unable to take advantage of the unique properties of laser diodes. The efficiency produced using the commercial laser resonator was quite low. With 405 mW from the diodes, the maximum output obtained with a highly reflective (HR) output mirror was 2.8 mW. Using a slightly lower reflectivity output mirror provided a slope efficiency of only 2.2%.
Thus, in accordance with this inventive concept a continuing need has been found in the state of the art for a high efficiency dye laser which is scalable to higher power, has a low threshold power, is compatible with liquid and solid state dye gain media, can be efficiently pumped by AlGaInP visible laser diodes and can operate either cw or pulsed, where pulsed operation can produce long or short pulses, and these pulses can be emitted between low and high pulse repetition rates.